[lit] Update lit.py shebang for Python3

[yln]

Summary

Update shebang to always use Python3 when executing lit.py directly.

A previous change of mine [1] revealed that we still use Python2 on some
bot configurations that invoke llvm/utils/lit/lit.py as a script
directly (instead of python3 path/to/lit.py).

[1] https://reviews.llvm.org/D94734

llvm/utils/lit/lit.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from lit.main import main
 
 if __name__ == '__main__':
     main()